The role of oxygen during the catalytic oxidation of ammonia on Co 3 O 4 (1 0 0)

Molecular modelling of the decomposition of NH3 over CoO(100)

“…The dehydrogenation energy barriers for NH 3 , NH 2 and NH on CoO(100) are 218 kJ mol À1 , 219 kJ mol À1 , and 210 kJ mol À1 higher, respectively, than those calculated on the Co 3 O 4 (100) surface [16]. The binding energies of NH 3 , NH 2 and NH on CoO(100) are 43 kJ mol À1 , 5 kJ mol À1 and 22 kJ mol À1 lower, respectively, than on the Co 3 O 4 (100) surface [16].…”

“…The binding energies of NH 3 , NH 2 and NH on CoO(100) are 43 kJ mol À1 , 5 kJ mol À1 and 22 kJ mol À1 lower, respectively, than on the Co 3 O 4 (100) surface [16]. Clearly, the dominant factor in the loss of catalytic activity of CoO towards NH x dehydrogenation compared to that of the Co 3 O 4 (100) surface is associated to the inability of lattice oxygen to assist the hydrogen abstraction process, rather than the stability of the NH x species at the surface.…”

“…The increase in binding energy as the NH 3 is being dehydrogenated has been reported on various transition metals [41,42] as well as on metal oxides [43,44]. The binding energy of H atop lattice O is 160 kJ mol À1 , which is 233 kJ mol À1 lower than the binding energy of H atop lattice O in the Co 3 O 4 (100) surface [16]. The lower binding energy of H on CoO compared to Co 3 O 4 is attributed to a significant vertical relaxation of lattice O upon binding of H in the CoO(100) surface.…”

“…On-site Coulomb correction to the 3d electrons of Co atoms was considered using the rotationally invariant formulation proposed by Dudarev et al [29] with a value of U eff ¼ (U À J) ¼ 3.3 eV, which was obtained by fitting the reaction energy of CoO oxidation to Co 3 O 4 [30]. We have shown in our previous work [16] that the PBE þ U approach with a U eff ¼ 3.3 eV provides geometry configurations and binding energies of NH x species on Co 3 O 4 (100) surfaces in good comparison with the hybrid functional HSE06 methods.…”

“…The high activity of Co 3 O 4 towards NH 3 oxidation has been ascribed to the participation of weak lattice surface oxygen sites in the hydrogen abstraction and NO formation leaving the surface with vacancy sites [16] and possible rapid O 2 adsorption to regenerate the O sites and its surface activity [16]. Lattice O in CoO surfaces is more coordinated than in Co 3 O 4 surfaces and therefore, in theory, less active for dehydrogenation than O in Co 3 O 4 .…”